This invention relates to the field of automatic transmissions for motor vehicles. More particularly, the invention pertains to a kinematic arrangement of gearing, clutches, brakes, and the interconnections among them in a power transmission.
When a motor vehicle is stationary for a period of time, such as while waiting at a traffic light, it is desirable to shut off the engine to save fuel. The engine must then be quickly re-started when the driver signals that he is ready to move again, usually by removing his foot from the brake pedal and applying pressure to the accelerator pedal. If the delay in delivering torque to the drive wheels is excessive, the driver will be unsatisfied with the vehicle. In order to minimize the delay, it is important that the transmission be prepared to transmit torque in first gear as soon as the engine is running. Traditionally, an automatic transmissions is adapted for this idle engine stop feature by adding an electrically driven pump to provide hydraulic pressure to engage the appropriate friction elements.
This invention avoids the need for an electrically driven pump by employing electro-mechanical actuation for the friction elements used in first gear. Electro-mechanical actuation is operable when the engine is not running. However, electromechanical actuation is limited to friction elements on the periphery of the gearbox, such as brakes, whereas hydraulic actuation can be utilized for friction elements that are located under rotating shells. With the kinematic arrangement of this invention, only brakes need to be applied to engage first gear.